306 research outputs found
Time-Contrastive Networks: Self-Supervised Learning from Video
We propose a self-supervised approach for learning representations and
robotic behaviors entirely from unlabeled videos recorded from multiple
viewpoints, and study how this representation can be used in two robotic
imitation settings: imitating object interactions from videos of humans, and
imitating human poses. Imitation of human behavior requires a
viewpoint-invariant representation that captures the relationships between
end-effectors (hands or robot grippers) and the environment, object attributes,
and body pose. We train our representations using a metric learning loss, where
multiple simultaneous viewpoints of the same observation are attracted in the
embedding space, while being repelled from temporal neighbors which are often
visually similar but functionally different. In other words, the model
simultaneously learns to recognize what is common between different-looking
images, and what is different between similar-looking images. This signal
causes our model to discover attributes that do not change across viewpoint,
but do change across time, while ignoring nuisance variables such as
occlusions, motion blur, lighting and background. We demonstrate that this
representation can be used by a robot to directly mimic human poses without an
explicit correspondence, and that it can be used as a reward function within a
reinforcement learning algorithm. While representations are learned from an
unlabeled collection of task-related videos, robot behaviors such as pouring
are learned by watching a single 3rd-person demonstration by a human. Reward
functions obtained by following the human demonstrations under the learned
representation enable efficient reinforcement learning that is practical for
real-world robotic systems. Video results, open-source code and dataset are
available at https://sermanet.github.io/imitat
The Murchison Widefield Array: Design Overview
The Murchison Widefield Array (MWA) is a dipole-based aperture array
synthesis telescope designed to operate in the 80-300 MHz frequency range. It
is capable of a wide range of science investigations, but is initially focused
on three key science projects. These are detection and characterization of
3-dimensional brightness temperature fluctuations in the 21cm line of neutral
hydrogen during the Epoch of Reionization (EoR) at redshifts from 6 to 10,
solar imaging and remote sensing of the inner heliosphere via propagation
effects on signals from distant background sources,and high-sensitivity
exploration of the variable radio sky. The array design features 8192
dual-polarization broad-band active dipoles, arranged into 512 tiles comprising
16 dipoles each. The tiles are quasi-randomly distributed over an aperture
1.5km in diameter, with a small number of outliers extending to 3km. All
tile-tile baselines are correlated in custom FPGA-based hardware, yielding a
Nyquist-sampled instantaneous monochromatic uv coverage and unprecedented point
spread function (PSF) quality. The correlated data are calibrated in real time
using novel position-dependent self-calibration algorithms. The array is
located in the Murchison region of outback Western Australia. This region is
characterized by extremely low population density and a superbly radio-quiet
environment,allowing full exploitation of the instrumental capabilities.Comment: 9 pages, 5 figures, 1 table. Accepted for publication in Proceedings
of the IEE
A new layout optimization technique for interferometric arrays, applied to the MWA
Antenna layout is an important design consideration for radio interferometers
because it determines the quality of the snapshot point spread function (PSF,
or array beam). This is particularly true for experiments targeting the 21 cm
Epoch of Reionization signal as the quality of the foreground subtraction
depends directly on the spatial dynamic range and thus the smoothness of the
baseline distribution. Nearly all sites have constraints on where antennas can
be placed---even at the remote Australian location of the MWA (Murchison
Widefield Array) there are rock outcrops, flood zones, heritages areas,
emergency runways and trees. These exclusion areas can introduce spatial
structure into the baseline distribution that enhance the PSF sidelobes and
reduce the angular dynamic range. In this paper we present a new method of
constrained antenna placement that reduces the spatial structure in the
baseline distribution. This method not only outperforms random placement
algorithms that avoid exclusion zones, but surprisingly outperforms random
placement algorithms without constraints to provide what we believe are the
smoothest constrained baseline distributions developed to date. We use our new
algorithm to determine antenna placements for the originally planned MWA, and
present the antenna locations, baseline distribution, and snapshot PSF for this
array choice.Comment: 12 pages, 6 figures, 1 table. Accepted for publication in MNRA
- …